1. Field of the Invention
The invention relates to an improvement in the process of preparing ethylene oxide by oxidation of ethylene with gaseous oxygen on a silver-based catalyst.
More particularly, the invention relates to the use of a silver-based supported catalyst, active in the oxidation process, and obtained by impregnation of a granular inert support with a decomposable silver salt, followed by drying and decomposition of the silver salt on the support, with the decomposition being carried out under specific conditions.
2. Description of the Prior Art
According to a process employed in the art, to a great extent ethylene oxide is obtained by contacting at high temperature ethylene with gaseous oxygen on a silver-based catalyst.
This catalyst usually consists of a granular inert support on which silver and possibly also minor quantities of additional metals such as platinum, gold, palladium, barium and calcium are deposited.
According to the process known in the art, the silver-based catalysts are prepared by impregnation of a subdivided inert support with a decomposable silver salt. Silver lactate is particularly suitable and can be used either a molten condition or a dissolved condition. The impregnated support is then dried and subsequently heat-treated at high temperature whereby the silver salt is decomposed by heat and metallic silver is fixed on the support.
The thermal decomposition (or activation of the catalyst) is normally carried out at a temperature from 250.degree. to 400.degree.C in an oxidizing atmosphere such as in the presence of air, or initially in a nitrogen atmosphere, and then in a nitrogen-oxygen atmosphere with a progressively increasing oxygen proportion.
According to another process known in the art, activation of the catalyst is still effected in a nitrogen-oxygen atmosphere with an increasing oxygen proportion, with the gases evolved on decomposition of the silver salt being caused to continuously flow through the catalyst particles. In this manner an influence is exerted up to a certain extent on the nature of the silver deposit on the support and, ultimately, on the properties of the catalyst.
According to what is known in the art, the nature of the silver deposit obtained from the decomposable salt and the adherence of the silver to the support are rather largely influenced by the carbon dioxide present in the gaseous atmosphere in contact with the catalyst being formed.
Therefore, according to a further known process, the decomposition of the silver salt is carried out in an oxidizing atmosphere which contains substantial quantities of carbon dioxide.
Further processes are moreover known for preparing active silver-based catalysts, with an activation step being carried out at high temperature in a non-oxidizing atmosphere or in a reducing atmosphere.